The field of the invention relates to a wireless round-trip-time (RTT) ranging operation between a relatively high-power, high-complexity interrogator device and a relatively low-power, low-complexity tag device. As illustrated in FIG. 1, the interrogator initiates the RTT ranging operation by sending a first wireless signal to the tag device. After receiving the first wireless signal, the tag device waits a predetermined amount of time and then sends a second wireless signal back to the interrogator device. The total round-trip-time is measured in the interrogator device between specific events (e.g., trailing edge) of the transmitted first wireless signal and received second wireless signal.
Since the duration of the turnaround time (TAT) and second wireless signal (T2) are known, the propagation time can be determined from the measured RTT:
                    τ        =                              RTT            -            TAT            -                          T              ⁢                                                          ⁢              2                                2                                    (        1        )            Range is proportional to propagation time, R=TC, where c is the speed of the wireless signals.
There may be other minor fixed delays associated with signal reception and processing, but these can be quantified and removed as well. The block diagram of FIG. 2 illustrates the basic components that comprise an interrogator device or a tag device. As previously mentioned, specific block characteristics such as processor complexity, clock speeds, transmit power, etc., are selected appropriately for a high-power interrogator and a low-power tag. A processor, which may be referred to as a controller, provides data waveforms “Dtx” to the modulator and receives data waveforms “Drx” from the demodulator. The processor also provides control information “Cntrl” to the Timer used to measure round-trip-time for ranging. Timer measurement data “Dtmr” is retrieved by the Processor. A frequency generator block produces various clock and oscillator signals used by the Processor, Timer, and Transceiver. A modulated signal “Stx” is converted to a wireless signal which is transmitted from the device, and a received wireless signal is converted to modulated signal “Srx” for further processing by the demodulator. In addition to recovering any information present in the received signal, the Processor may also include other functions used to determine arrival time of the received signal.
Multipath Issues
One of the issues with implementing a wireless RTT ranging system is multipath propagation, in which radio waves travel by different paths and reach the receiver with small time offsets. As can be seen in FIG. 3, in a simplified system block diagram, a tag device and an interrogator device may be in communication via communication link having multiple paths P1, P2 and P3. While the range on the direct path P1 is usually desired in most applications, delayed copies of the transmitted signal may also reach the tag via alternative paths P2 and P3. A RTT ranging determination using the resulting composite received signal therefore contains multipath error, which must be removed to produce the correct ranging value along the direct path P1. A means of compensating or correcting for this error, compatible with a low-power, low-complexity device such as a tag, would therefore be useful.